Both during a tank refuelling procedure and also while a vehicle in which a tank is used is in operation and stationary, it is necessary to ensure that fuel vapor above the level of fuel in the fuel tank is appropriately discharged so that the fuel tank can be filled with fuel or so that an unacceptably increased pressure is not generated in the tank. In the case of a tank refuelling procedure a volume of gas of up to 60 liters per minute may typically be displaced from the tank by the incoming flow of fuel. The displaced fuel usually involves a mixture of gaseous hydrocarbons and air. Likewise, during normal operation of a motor vehicle, gaseous hydrocarbons are given off, which under certain operating conditions could result in an unacceptable rise in the pressure in the fuel tank. As is known, an increase in temperature increases the tendency on the part of the fuel to change from the liquid phase to the gaseous phase. That phenomenon is further promoted by shaking and rolling movements of the fuel tank. The accumulation of gas in the fuel tank, induced by temperature and/or movement, is usually removed from the fuel tank by way of operational venting conduits, in which case the volatile hydrocarbons are then deposited in a fuel vapor filter which is typically in the form of an activated carbon filter. The fuel vapor filter should be so designed that virtually completely purified air is discharged from the filter to the ambient atmosphere.
In terms of refuelling a motor vehicle, two different forms of venting are known, more specifically in Europe the very widespread procedure involves suction removal of the displaced volume of gas at the refuelling gun, in which case only a part of the displaced volume of gas is passed to the fuel vapor filter, while in the USA the procedure involves completely removing the volume of gas produced by way of the fuel vapor filter which is fitted in the vehicle.
At any event residual emissions of hydrocarbons reach the atmosphere through the fuel vapor filter, and that is basically detrimental for reasons relating to the emission of pollutants.
In this respect reference may be made to U.S. Pat. No. 4,829,968 in which fuel vapor from the free volume in the fuel tank above the level of fuel therein is caught in a collecting container provided with an activated carbon filling and a heating element. The hydrocarbons which are adsorbed by the activated carbon are fed in the gaseous phase under pressure back to the fuel again by way of a sparger discharge unit disposed below the level of the liquid. Within the collecting container which includes a fuel vapor filter air and liquid hydrocarbons are separated in the usual manner by deposit of the hydrocarbons at the activated carbon. The air which is cleared of hydrocarbons in that way is discharged to the atmosphere.
It will be seen therefore that the problem of pollution of the atmosphere by residual emissions from the fuel vapor filter still applies in this case.